The present invention relates to a die head assembly for a meltblown apparatus, and more particularly to a process and breaker plate assembly for producing bicomponent fibers in a meltblown apparatus.
A meltblown process is used primarily to form fine thermoplastic fibers by spinning a molten polymer and contacting it in its molten state with a fluid, usually air, directed so as to form and attenuate filaments or fibers. After cooling, the fibers are collected and bonded to form an integrated web. Such webs have particular utility as filter materials, absorbent materials, moisture barriers, insulators, etc.
Conventional meltblown processes are well known in the art. Such processes use an extruder to force a hot thermoplastic melt through a row of fine orifices in a die tip head and into high velocity dual streams of attenuating gas, usually air, arranged on each side of the extrusion orifice. A conventional die head is disclosed in U.S. Pat. No. 3,825,380. The attenuating air is usually heated, as described in various U.S. Patents, including U.S. Pat. No. 3,676,242; U.S. Pat. No. 3,755,527; U.S. Pat. No. 3,825,379; U.S. Pat. No. 3,849,241; and U.S. Pat. No. 3,825,380. Cool air attenuating processes are also known from U.S. Pat. No. 4,526,733; WO 99/32692; and U.S. Pat. No. 6,001,303.
As the hot melt exits the orifices, it encounters the attenuating gas and is drawn into discrete fibers which are then deposited on a moving collector surface, usually a foraminous belt, to form a web of thermoplastic material. For efficient high speed production, it is important that the polymer viscosity be maintained low enough to flow and prevent clogging of the die tip. In accordance with conventional practice, the die head is provided with heaters adjacent the die tip to maintain the temperature of the polymer as it is introduced into the orifices of the die tip through feed channels. It is also known, for example from EP 0 553 419 B1, to use heated attenuating air to maintain the temperature of the hot melt during the extrusion process of the polymer through the die tip orifices.
Bicomponent meltblown spinning processes involve introducing two different polymers from respective extruders into holes or chambers for combining the polymers prior to forcing the polymers through the die tip orifices. The resulting fiber structure retains the polymers in distinct segments across the cross-section of the fiber that run longitudinally through the fiber. The segments may have various patterns or configurations, as disclosed in U.S. Pat. No. 5,935,883. The polymers are generally xe2x80x9cincompatiblexe2x80x9d in that they do not form a miscible blend when combined. Examples of particularly desirable pairs of incompatible polymers useful for producing bicomponent or xe2x80x9cconjugatexe2x80x9d fibers is provided in U.S. Pat. No. 5,935,883. These bicomponent fibers may be subsequently xe2x80x9csplitxe2x80x9d along the polymer segment lines to form microfine fibers. A process for producing microfine split fiber webs in a meltblown apparatus is described in U.S. Pat. No. 5,935,883.
A particular concern with producing bicomponent fibers is the difficulty in separately maintaining the polymer viscosities. It has generally been regarded that the viscosities of the polymers passing through the die head should be about the same, and are achieved by controlling the temperature and retention time in the die head and extruder, the composition of the polymers, etc. It has generally been felt that only when the polymers flow through the die head and reach the orifices in a state such that their respective viscosities are about equal, can they form a conjugate mass that can be extruded through the orifices without any significant turbulence or break at the conjugate portions. When a viscosity difference occurs between the respective polymers due to a difference in molecular weights and even a difference in extrusion temperatures, mixing in the flow of the polymers inside the die head occurs making it difficult to form a uniform conjugate mass inside the die tip prior to extruding the polymers from the orifices. U.S. Pat. No. 5,511,960 describes a meltblown spinning device for producing conjugate fibers even with a viscosity difference between the polymers. The device utilizes a combination of a feeding plate, distributing plate, and a separating plate within the die tip.
There remains in the art a need to achieve further economies in meltblown processes and apparatuses for producing bicomponent fibers from polymers having distinctly different viscosities.
Objects and advantages of the invention will be set forth in the following description, or may be apparent from the description, or may be learned through practice of the invention.
The present invention relates to an improved die head assembly for producing bicomponent meltblown fibers in a meltblown spinning apparatus. It should be appreciated that the present die head assembly is not limited to application in any particular type of meltblown device, or to use of any particular combination of polymers. It should also be appreciated that the term xe2x80x9cmeltblownxe2x80x9d as used herein includes a process that is also referred to in the art as xe2x80x9cmeltspray.xe2x80x9d
The die head assembly according to the invention includes a die tip that is detachably mounted to an elongated support member. The support member may be part of the die body itself, or may be a separate plate or component that is attached to the die body. Regardless of its configuration, the support member has, at least, a first polymer supply passage and a separate second polymer supply passage defined therethrough. These passages may include, for example, grooves defined along a bottom surface of the support member. The grooves may be supplied by separate polymer feed channels.
The die tip has a row of channels defined therethrough that terminate at exit orifices or nozzles along the bottom edge of the die tip. These channels receive and combine the first and second polymers conveyed from the support member.
An elongated recess is defined in the top surface of the die tip. This recess defines an upper chamber for each of the die tip channels. A plurality of elongated breaker plates are disposed in a stacked configuration within the recess. The uppermost breaker plate has receiving holes defined therein to separately receive the polymers from the supply member passages. For example, in one embodiment of the uppermost breaker plate, alternating receiving holes are disposed along the upper surface of the breaker plate to separately receive the two polymers. In this embodiment, the receiving holes may be in fluid communication with distribution channels defined in the bottom of the upper breaker plate. These distribution channels are disposed so as to separately distribute the two polymers to an adjacent breaker plate. In one particular embodiment, these distribution channels are disposed across the breaker plate, or transverse to the longitudinal axis of the breaker plate. One set of the distribution channels extends about halfway across the breaker plate so as to distribute one of the polymers to a row of holes in the adjacent breaker plate. Another set of the distribution channels extends generally across the breaker plate so as to distribute the other polymer to at least one other row of holes in the adjacent breaker plate.
The remaining breaker plates have holes or channels defined therethrough configured to divide the polymers distributed by the upper breaker plate into a plurality of separate polymer streams and to direct these polymer streams into the die tip channels. Thus, at each die tip channel, the first and second polymers are conveyed from the support member supply passages, through the breaker plates, and into the die tip channels as a plurality of separate polymer streams corresponding to the number of holes in a lowermost breaker plate. The polymer streams combine in the channels prior to being extruded from the orifice as bicomponent polymer fibers.
A filter element, such as a screen, is disposed in the recess so as to separately filter the polymer streams prior to the streams being conveyed into the die tip channels. For example, this filter screen may be disposed between the bottom two breaker plates.
In one particular embodiment of the invention, three stacked breaker plates are disposed in the die tip recess and include an upper breaker plate, a middle breaker plate, and a lower breaker plate. The lower breaker plate has a grouping of holes defined therethrough at each of the die tip chambers. Thus, the lower breaker plate has a series of such groupings defined longitudinally therealong, wherein one such grouping is provided for each die tip channel. The invention is not limited to any particular number or configuration of holes defined in the lower breaker plate. For example, in one embodiment, three such holes are provided for each grouping and divide the polymers into three separate polymer streams that are combined in the die tip channels.
In the embodiment of the invention wherein three breaker plates are provided, the middle breaker plate may have a plurality of holes defined therethrough that are disposed relative to the distribution channels in the upper breaker plate so that each of the polymers is distributed to at least one of the holes in the middle breaker plate, and each of the middle breaker plate holes receives only one polymer. Thus, the polymers are not mixed in the middle breaker plate holes, and at least one of the middle breaker plate holes is used to separately convey one of the polymers. Each of the lower breaker plate holes of each grouping of holes is in fluid communication with one of the middle breaker plate holes such that each of the polymers is separately distributed to at least one of the lower breaker plate holes, and each of the lower breaker plate holes receives only one polymer. The number of lower breaker plate holes determines the number of separate polymer streams extruded into the die tip channels.